The present disclosure relates generally to a power supply with galvanic isolation, and more particularly, to methods and apparatuses for transmitting information from a secondary side to a primary side where the primary and secondary sides are galvanically isolated from each other.
As mobile devices are becoming more popular to the world, users rely on them and demand longer time for use before charging the next time. Thus, mobile devices are required to be more power saving, have batteries with larger capacity, and be capable of being faster charged.
Several methods for quickly charging the batteries in mobile devices have been provided. Qualcomm, for example, has announced “Quick Charge”™ 2.0. A Quick Charge enabled device sends differential signals to a charger, which according alters its output voltages for fast charging the Quick Charge enabled device. Generally speaking, the higher output voltage the faster charging.
A charger 10 is demonstrated in FIG. 1, capable of fast charging an electric device connected to a USB connector 12. In consideration of safety, the charger 10 has a primary side 14P and a secondary side 14S galvanically isolated from each other. In other words, there is no direct current connection between the primary side 14P and the secondary side 14S. The circuitry at the primary side 14P is majorly powered by input power source VIN and input ground line GNDI, both of which are generated from bridge rectifier 16 rectifying an alternating current (AC) power source VAC-IN from a power grid. A secondary winding LS at the secondary side 14S could demagnetize to provide output power source VOUT and output ground line GNDO.
A secondary-side controller 18 receives differential signal from lines D+ and D− of the USB connector 12, to set an operation mode the primary-side controller 20 operates in. Accordingly, primary-side controller 20 controls power conversion so as to determine the voltage rating of the output power source VOUT. The voltage rating refers to a voltage level that the output power source VOUT is controlled to approach. In FIG. 1, the secondary-side controller 18 generates a pulse-width modulation (PWM) signal SD to drive emitter 26E of a photo coupler 26. Receiver 26R of the photo coupler 26 and a low-pass filter 24 accordingly generate a direct current (DC) signal at node R, whose voltage level accordingly corresponds to the duty cycle of PWM signal SD. A translator 22 converts the DC signal at node R to provide signals setting the primary-side controller 20.
In case that the differential signal from lines D+ and D− demands the voltage rating of the output power source VOUT to be 12V, for instance, the secondary-side controller 18 provides the PWM signal SD with a duty cycle of 50%. As a result, the DC voltage at node R is 2.5V, which causes the translator 22 to make the primary-side controller 20 regulating the output voltage VOUT of the output power source VOUT at 12V. Similarly, if the differential signal from lines D+ and D− demands the voltage rating of the output power source VOUT to be 20V, the duty cycle of the PWM signal SD becomes almost 100%, the DC voltage at node R 5V, so the output voltage VOUT of the output power source VOUT starts to approach 20V.